Disc-shaped optical recording media (each hereinafter referred to simply as optical disc) such, for example, as a CD (Compact Disc), a DVD (Digital Versatile Disc) and a BD (Blu-ray Disc: trademark registered) have widely spread. In the standards of these optical discs, a DPD (Differential Phase Detection) scheme is widely employed as a tracking error detection scheme for a playback-dedicated ROM disc.
FIG. 10 is an explanatory drawing of a tracking error detection technique based on the DPD scheme. In the DPD scheme, as first illustrated in FIG. 10A, a light receiving unit in which four light receiving regions of A, B, C and D are dividedly formed is used as a light receiving unit for receiving reflected light from an optical disc. Herein, in such a quadruple light receiving unit, the regions A, B, C and D are formed by segmentation with a division line that extends in the longitudinal direction of a track (linear direction) and a division line that extends in the short-side direction thereof (disc radial direction) on the basis of the track, as the reference, formed of pits (shaded parts in the figure) lined up on the optical disc. Specifically, in the DPD scheme, each of a group of the regions A and B and a group of the regions C and D is a group obtained by segmentation with the division line in the track longitudinal direction and each of a group of the regions A and D and a group of the regions B and C is a group obtained by segmentation with the division line in the track short-side direction. Moreover, when an upstream side and a downstream side are defined on the basis of the direction, as the reference, of advancement of the pits in accordance with rotation of the optical disc, the group of the regions A and B is arranged on the upstream side and the group of the regions C and D on the downstream side.
FIG. 10A schematically illustrates a situation of transition in a pit passing through relative to the light receiving unit (time points t1 to t5). At time point t1, there is illustrated a state where the heading edge part of the pit reaches the vicinity of the upstream side end part of the light receiving unit, at time point t2, a state where the heading edge part reaches the vicinity of the track short-side direction division line of the light receiving unit, at time point t3, a state where the heading edge part reaches the vicinity of the downstream side end part of the light receiving unit, at time point t4, a state where the ending edge part of the pit reaches the vicinity of the track short-side direction division line of the light receiving unit, and at time point t5, a state where the ending edge part reaches the vicinity of the downstream side end part of the light receiving unit. In the left of the plane, there is illustrated a situation of the beam spot detracking on the left side relative to the track center, in the center thereof, a situation of the beam spot tracing the track center, and in the right thereof, a situation of the beam spot detracking on the right side relative to the track center.
In the DPD scheme, on the basis of the light reception results of the four regions A to D in the light receiving unit, a signal (A+C) and a signal (B+D) are generated and a phase difference between these (phase difference arising in the light receiving unit due to optical interference) is detected to generate a tracking error signal TES. Notably, the signal (A+C) means the addition result of a signal A and a signal C generated on the basis of the light reception results of the respective regions A and C and the signal (B+D) means the addition result of a signal B and a signal D generated on the basis of the light reception results of the respective regions B and D.
FIG. 10B illustrates waveforms of the signal (A+C) and the signal (B+D) obtained during time points t1 to t5 in the occasion of detracking on the left side (in the left of the plane), in the occasion of tracing the track center (in the center of the plane) and in the occasion of detracking on the right side (in the right of the plane) illustrated in FIG. 10A, respectively. As illustrated in the figure, in the occasion of tracing the track center, a phase difference does not arise between the signal (A+C) and the signal (B+D). In the occasion of detracking on the left side, a phase difference arises such that the phase of the signal (A+C) advances, and conversely, in the occasion of detracking on the right side, a phase difference arises such that the phase of the signal (B+D) advances.
FIG. 10C illustrates a situation of generating the tracking error signal TES from the signal (A+C) and the signal (B+D) on the basis of the DPD scheme. As illustrated in the figure, the phase difference between the signal (A+C) and the signal (B+D) is detected, including information of its polarity (which phase is advanced/delayed) and the tracking error signal TES is generated on the basis of the result. The figure illustrates examples of waveforms obtained in the occasion of the beam spot having passed through the track center from the left side to the right side for the individual signals of the signal (A+C) and the signal (B+D), the polarity signals of these, the phase difference detection signal between the signal (A+C) and the signal (B+D) (including the polarity information), and the tracking error signal TES generated from the phase difference detection signal.
FIG. 11 illustrates a phase comparator, by way of example, essential in the case where the DPD scheme is employed. The phase comparator illustrated in FIG. 11 is a so-called EXOR (EX-OR: EXclusive OR) phase comparator. As illustrated in the figure, the EXOR phase comparator is at least provided with an EXOR circuit and a flip-flop to which the binarized signal (A+C) and the binarized signal (B+D) are respectively inputted, two AND gate circuits and an operational amplifier.
The output from the EXOR circuit in the FIG. is “1” when two input signals are different from each other and is “0” when they are same. The flip-flop in the bottom discriminates the difference, between the signals, detected by the EXOR circuit to represent phase advancement or to represent phase delay, determining the polarity of the phase comparator output.
Use of such a phase comparator, for example, enables the tracking error signal TES to be generated on the basis of the DPD scheme as described using FIG. 10C above.